4. JETS IN RADIO GALAXIES AND QUASARS

X-ray jets in a diverse collection of objects has become possible as a
distinct field of research only with the 100-fold improvement in
2-dimensional imaging afforded by the 0.5" resolution of
Chandra, compared to the previous 5" imaging of the
Einstein and ROSAT observatories. Jets are studied in
contexts as varied as the symbiotic binary R Aqr
(Kellogg (2001)),
the galactic black hole candidate XTE J1550-564
(Corbel 2002),
plerions (discussed in Section 2), low
power FR I sources
where the X-ray emission is interpreted as an extension of the radio
synchrotron emission (e.g.,
Worrall (2001)),
and in powerful FR II
galaxies and quasars. This section will consider only the last topic.

In Figure 8 I select some examples which
illustrate the
X-ray emission at the hotspots at the end of jets. In all these cases,
from the radio observation of flux and spatial extent, plus the
assumption of equipartition, one can conclude that the synchrotron
photon density is the largest energy density in the X-ray emitting
region. Thus the natural X-ray mechanism is synchrotron self-Compton
(SSC) emission. The X-ray fluxes are generally consistent with
magnetic fields in the range 70 to 320 µGauss, which are
just a little
below the equipartition field values (see refs. given in figure
caption). This is a body of evidence that conditions near
equipartition might be a reasonable assumption.

Figure 9 shows some of the X-ray images of jets
(in false color), overlaid with radio contours. These are samples from two
surveys, one by
Sambruna (2002),
the other by
Marshall (2002),
(also reported in
Schwartz
(2003)).
We also see the discovery image of PKS 0637-752
(Schwartz
(2000),
and a radio-optical-X-ray view of the jet in 3C 273
(Marshall
(2001)).
The X-ray emitting regions closely follow the radio in general, but
the intensities sometimes correlate closely, as in the straight western jet
of PKS 0637-752, and sometimes anticorrelate as in 3C 273. We will
use the four PKS objects from our survey (bottom of
Figure 9) to discuss the physical conditions in jets
(Schwartz
(2003b)).

We have 8.64 GHz ATCA images of all our sources, and in some cases 4.8 GHz
data. We smooth both the radio and X-ray to a 1.2" resolution, and
superpose them by forcing coincidence of the quasar cores. We then
divide the jets into distinct regions. This is somewhat subjective,
guided by the features in the radio and X-ray emission. To label
the different regions we use the term "knots" (K), with numbers
increasing away from the quasar, but we don't intend this to prejudice
the nature of the actual structure.

Figure 10 shows the spectral energy
distributions we
construct for each region. Optical upper limits from Magellan
observations (J. Gelbord, private communication and in preparation
2004) directly show that for most cases the X-ray emission cannot be a
simple extension of the radio synchrotron spectrum. For other regions,
e.g., K3 of PKS 0208-512, the radio spectral shape would not connect
to the X-ray region. For some other regions, e.g., K2 of PKS 0920-397
and K2 of PKS 0208-512 the X-ray emission could well result from a
continuation of the relativistic electrons to high enough energy to
emit X-ray synchrotron emission, as inferred for the first knot, A1,
in 3C 273
(Marshall
(2001)).
The simplest X-ray emission mechanism,
given the strong correlation with the radio, should invoke radiation
from the same spectrum of relativistic electrons. This indicates some
form of inverse Compton (IC) emission. From the size and radio emission,
we know that SSC will not be important, and at 10's to 100's of kpc
from the quasar the energy density of photons from the central black
hole will not give significant radiation.

The most likely target photons for IC emission are the cosmic
microwave background (CMB). This was originally discussed by
Felten &
Morrison (1966)
in the context of explaining the cosmic X-ray
background. However, in the original case of PKS 0637-752, calculating
the equipartition magnetic field gave a result about 100 times larger
than the maximum magnetic field which would allow the X-rays to be
produced by IC/CMB radiation. The problem of requiring total energies
more than 103 times larger was resolved by
Tavecchio
(2000) and
Celotti (2001),
who considered the enhancement of the apparent CMB density by the factor
2,
(Dermer &
Schlickeiser (1994)),
in a frame moving with bulk relativistic velocity
= (1 - 1 /
2)1/2 with
respect to the CMB frame. If one plots the required relativistic
beaming factor =
((1 -
cos))-1
against the required rest frame magnetic field, then since
BIC
and 1 /
Beq, one can always find a
solution for and B for
which the source is near equipartition
in its rest frame, and the same population of electrons produces radio
synchrotron radiation in the B field, and X-ray inverse Compton
radiation off the cosmic microwave background.

We will continue by assuming all the jet emission is due to
IC/CMB. Figure 11 (left panel) shows the range
of Doppler factors, 2 to 10, and
intrinsic magnetic fields, B
5 to 25
µG, for these objects
(Schwartz
(2003b)).
In this derivation, we have had to assume that
=
, since we do not have
independent information on the orientation of the jet. From
, we can infer a
maximum angle of the jet from our line of sight,
max =
cos-1[( - 1 /
) /
(2 -
1)1/2], which we use to
compute the minimum space distance (3 dimensions) of each region from
the central quasar. We estimate (right panel of
Figure 11) the kinetic flux of each element as
A2cU
(e.g.,
Ghisellini
& Celotti (2001)),
where A is the cross sectional area (and
we assume cylindrical symmetry), and U the total energy density in the
rest frame of the jet. We assume equipartition, with an equal energy
in protons and electrons, so that U = 3 B2 /
(8 ). Under all
these assumptions, the lines
1/B are the lines of
constant kinetic flux, as shown in the left panel.

Figure 11. Structure of the X-ray
jets. Left panel shows the Doppler
factors and rest frame magnetic fields inferred for each
region. Uncertainties are systematics dominated and about a factor
of 2. Solid lines show the loci of constant kinetic flux. Right
panel plots the kinetic flux through each region, vs. the minimum
space distance of each component. Colored triangles, diamonds,
circles and squares indicate the same source in each frame. Crosses
plot the bolometric radiative luminosity of the quasar.

In the right panel, the crosses plot the bolometric radiative
luminosity of the quasar cores. We see that the kinetic flux in the
jet is comparable to or greater than the accretion flux. This is
consistent with the conclusion of
Meier (2003)
that accretion flow models must also consider jet production.

One of the most dramatic implications of the inference that we see
IC/CMB X-radiation from radio jets is that any given object would
appear to have a constant X-ray surface brightness even as it were
displaced to an arbitrarily large redshift
(Schwartz
(2002)).
This is because the energy density of the CMB increases as (1 +
z)4, exactly offsetting the (1 + z)-4 cosmological
diminution of surface
brightness. Since the observed X-ray jet structures have
length scales of 10's of kpc projected on the sky, they will be
at least several arcsec long at redshifts greater than 2, and would
easily be resolved by Chandra. In fact, all objects intrinsically
similar to PKS 0637-752, or the outer knots of 3C 273, would be bright
enough to already be detected by ROSAT, but would appear as
point sources to the resolution of the PSPC all-sky survey.

Where are these bright X-ray jets at high redshifts? They could not be
recognized as extended in the ROSAT all sky survey, so they
would most likely be identified simply as part of the quasar core
emission. If the quasar itself were not recognized, e.g., because it
was too faint, the jet could be among the miscellaneous unidentified
sources. Alternately, the jet could outshine the quasar in X-rays,
and be cataloged at a position some distance away from the quasar, and
again be a miscellaneous unidentified source.

Figure 12 shows a possible example of the last
case. In
this ROSAT HRI pointed observation, there was only a possible
3 detection of the
quasar GB 1713+2148
(Vignali (2003)).
There is an obvious stronger, point source about 1 arcmin to the NW
(left panel of Figure 12). When the NVSS 1.4
GHz image
(Condon (1998))
is superposed, we see the contours of the radio loud
quasar extending around this source
(Gurvits (2003)),
so that it is
clearly associated with the quasar, at a redshift z = 4.011
(Hook & McMahon
(1998)).
We still need a high resolution X-ray image to
ascertain if this is really an X-ray jet, or perhaps just a hotspot or
lobe.

Figure 12. Left panel shows a ROSAT
HRI observation of the quasar GB 1713+2148 (green cross). The quasar is
at most a
3
detection in the 20" extraction circle. A stronger,
unidentified source lies
1' to the NW. The right
panel superposed the NVSS 1.4 GHz radio contours, clearly showing
the connection of the stronger source to the quasar.

Another case is due to
Siemiginowska (2003)
(Figure 13). Here the X-ray image (left panel)
is clearly extended,
and the analysis in the central panel shows that the data (solid line)
cannot be simulated simply by two point sources (dotted line).
Cheung (2003)
has analyzed archival VLA data, and found a
coincident jet at 1.4 GHz (right panel). An IC/CMB analysis shows
that this jet must be in relativistic motion, with a Doppler factor
2.6 and a magnetic field
B 161µG, where the
uncertainty is largely due to the uncertain slope of the radio and
X-ray emission
(Siemiginowska (2003)).

Figure 13. Left and center from
Siemiginowska
(2003):
Chandra image binned in 0.15." pixels, clearly
showing extent to the WSW of the quasar. The circle is 2.5"
diameter, and contains 95% encircled power. The solid histogram
is the intensity profile along the jet, which clearly cannot be
simulated by two point sources (dotted histogram). Right:
Cheung (2003)
subsequently discovered a radio jet in archival 1.4
GHz VLA data.